If there is useful information for law enforcement unrelated to the case at hand, it seems that we have plenty of already-existing mechanisms. They could offer him immunity in exchange for unlocking the drive, thus completely skirting the fifth amendment issue, or they could convict him based on other evidence, and offer favorable sentencing in exchange for unlocking the drive.

The fact that he is being compelled to offer evidence against himself seems like the dubious part of this whole proceeding.